This is the most critical period for the babys health as most of the organs develop during the first three months. Any disease, exposure to x-rays or radiation, use of tobacco, alcohol or use of medicines during this period can seriously harm babys health. Also you must take adequate rest and nutritious diet during this time. Avoid taking medicines, even those available without prescription or shown on TV. If absolutely essential, take them only under direct supervision of your doctor. Inform him/her that you are pregnant (or even that you may be) so that he/she can prescribe only medicines, known to be safe during the 1st trimester.
Detailed below is the normal development during each week of the first trimester.